Permeability of electrotonic junction, between embryonic cells of Fundulus embryos is being characterized by intracellular injection of tracers. Junctions between adult cells are generally less permeable than those between embryonic cells, and the time of appearance of the adult characteristics is under investigation. Permeability of electrotonic junctions is being studied in the septate axon of the crayfish by similar methods. Inhibitors of cytoplasmic movements are being evaluated for effects on junction formation and dissolution.